Multiple modalities currently exist to image the ocular anterior segment of an eye. These imaging devices aid in the clinical diagnosis and care of ocular disease. Optical coherence tomography (“OCT”) is one such technique used to image in vivo the ocular anterior segment of an eye in a non-invasive fashion. Specific implementations can comprise time-domain OCT systems (such as those sold under the name of VISANTE® by Carl Zeiss Meditec, Inc.), and Fourier-domain OCT systems, including both swept-source and spectrometer-based spectral-domain (“SDOCT”) implementations. Many previously disclosed anterior segment OCT systems have utilized illumination light in the 1310 nm region. Recently, several SD-OCT systems have been developed that utilize an 840 nm light source and are designed for imaging the retina, but that can also be modified with an adapter or a separate patient interface to image the ocular anterior segment of an eye. These SD-OCT systems offer the advantage of higher axial resolutions and faster scanning over prior systems.
All OCT imaging systems are subject to the effects of refraction at surfaces corresponding to interfaces between regions of differing refractive index within a sample, including between the air and the surface of the sample as well as internal sample interface surfaces. For a sample such as the anterior segment of an eye, important refractive index interfaces comprise the outer, herein referred to as epithelial, and inner, herein referred to as endothelial surfaces of the cornea, as well as the outer and inner surfaces of the crystalline lens. Additionally, for samples containing regions of different refractive index, it can be important for images acquired of the sample to reflect the true physical dimensions of the sample rather than to be distorted by the varying speed of light in different sample regions. Both of these potential pitfalls can be particularly important in applications such as corneal biometry where accurate measurements of various clinically significant parameters must be computed from the image data. Such computations are very sensitive to even small image errors due to refraction or distortion. Most current OCT systems do not correct the raw image data for refraction at sample interfaces or for the effects of different sample regions having differing refractive indices. Most current OCT systems instead assume that the light incident on the sample continues in a straight line through the sample and thus plot the raw image A-scan data corresponding to a depth-resolved reflectivity map of the sample on this assumed undeviated path. These systems also do not correct for the effects of different refractive indices in different regions of the sample. At best, these systems may divide the observed A-scan data by some assumed average index of refraction of the entire sample. As such, raw OCT data in current generation OCT systems does not accurately represent the true position of internal sample structures and are thus not able to support calculation of clinically significant parameters which depend on accurate image data. In particular, to produce accurate quantitative measurements of structures of the ocular anterior segment, accounting for the effects of refraction of the sample arm light and for the effects of differing indices of refraction in different sample regions is required.
Prior methods to correct for refraction in OCT images have been developed. They do not, however, account accurately or completely for correction of volumetric, three-dimensional (“3D”) OCT datasets. One method is limited to two-dimensional (“2D”) processing, which assumes that refraction occurs only within the plane of individual acquired B-scans (defined as sets of A-scans acquired along a straight line comprising a cross-sectional image of the sample). For a curved 3D structure such as the cornea, 2D refraction correction is correct only if the sample is rotationally conically symmetric about some axis passing through the apex of the sample, and if the acquired B-scan data passed exactly through the apex point. The first condition is rarely true for realistic samples such as the human cornea, especially if they have been modified surgically. The second condition is true only for idealized radial scan patterns, which may not be optimal because they oversample the central region and undersample the outer region and in any case are difficult to obtain correctly due to unavoidable patient motion or operator misalignment of the OCT system.